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It's much easier to add whole pounds in our head instead of adding pounds and pence.

We can use our knowledge of rounding numbers to help us estimate costs.

Joe orders a slice of cake costing £3.80.

What is £3.80 rounded to the nearest pound?

We're going to use a number line to see if £3.80 should be rounded up or rounded down.

This number line has been labelled with the nearest pounds on either end and each mark on the number line represents 10p.

To round £3.80 to the nearest pound we place it on a number line.

Whichever whole number it is closer to on the number line is the number we round it to.

You could also look at the tenths column to decide if you should round up or round down.

• If the number in the tenths column is 5 or bigger then you round up.
• If the number in the tenths column is 4 or less then you round down.

There are 8 tenths in £3.80 so we round up. If Joe's cake had cost £2.49 or less we would round down.

Knowing how to estimate costs is useful when we are shopping and need to check if we have enough money.

Joe is shopping for food for his packed lunch and this is the cost of items in his local supermarket.

What is the cost of the cheese and the cake to the nearest pound?

For both of these items we round up because they both have tenths above 5.

• Cheese = £4
• Eggs = £2

Now we’ve rounded the cost of the cheese and eggs to the nearest pound, what is the approximate total cost of the cheese and eggs?

4 +2 = £6

Estimating costs can also help us work out how much change we'll receive.

Joe wants to buy strawberries, crisps and melon.

Approximately how much change will he receive if he pays with £20?

• Round the items to the nearest pound
• Add the approximate cost (the rounded numbers) of all of the items together
• Subtract this from 20

Is your answer £10?

Practise rounding to the nearest pound.